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Healthy mice have been cloned from cells from dead mice that had been frozen for 16 years, raising the possibility that endangered species could be cloned from old carcasses that have been tossed in freezers, rather than from living cells frozen using elaborate techniques.

The finding also raises hopes of one day being able to resurrect extinct animals frozen in permafrost, such as the woolly mammoth, says Teruhiko Wakayama of the RIKEN Center for Developmental Biology in Kobe, Japan, who led the research. "It would be very difficult, but our work suggests that it is no longer science fiction," he says.

The Wakayama team used a modified version of a cloning technique in which the nucleus of a mouse cell – in this case a cell from dead tissue that has been frozen and then thawed – is injected into a mouse egg that has had its nucleus removed. The resulting embryo was then used to create embryonic stem cells, capable of generating every cell type in the body, and the nuclei of these cells were injected into other eggs to produce clones.

In a surprise finding, the Wakayama team discovered that it was easiest to create clones from brain tissue, even though clones have never been created from living brain cells. Wakayama speculates that freezing and thawing the tissue somehow makes it easier to “reprogram” the brain cell nucleus. Brain tissue is also high in sugars, which can protect cells when they freeze. This may explain why the DNA remained undamaged, says Wakayama.

Other teams have already cloned mice from previously frozen dead cells. But this is the first time animals have been cloned from lumps of tissue frozen without the use of chemicals that might protect the cells from damage.

The gene bank in the freezer

Globally, there are several cloning programmes that aim to increase the size of rapidly-dwindling populations of endangered species such as African wildcats, and maintain genetic diversity through one-off clonings of individuals that haven't bred. These programs depend on the animal cells undergoing specialised chemical procedures before being frozen so that they come to life when thawed.

Many zoos are not in a position to collect cells and freeze them in such a way as to preserve their viability, says Robert Lanza of Advanced Cell Technology in Worcester, Massachusetts, but they can put a dead animal "in a plastic bag and throw it in the freezer", he adds. "With a kitchen freezer you could store the genetic diversity of every panda in existence."

Despite the excitement surrounding the technique, more research will be needed before it can be used on endangered species, says Martha Gomez of the Audubon Nature Institute in New Orleans, Louisiana."For instance, we don't know how long we can keep bodies of different species frozen and still get viable DNA. It may depend on size and species," she says.

What's more, most conservationists agree that cloning should be considered only as a last resort for species such as the northern white rhino, where all other attempts at conservation have failed, says Paul Bartels, manager of BioBankSA at the National Zoological Gardens of South Africa in Pretoria. Nonetheless, he says, he will be asking biologists to start freezing the bodies of endangered species that have died.

"We intend to bring this [finding] to the attention of as many biologists working on endangered species as possible, through circulars, e-mail, newsletters, and talks," he says.

A mammoth challenge

Resurrecting extinct animals would be far trickier. Woolly mammoth carcasses would most likely have frozen and thawed several times over the aeons, which would cause far more damage to the nucleus than a one-off freezing.

Potentially easier would be cloning cryogenically frozen humans, though the consensus among cloning experts is that it would be unethical and dangerous to clone a human. In any case, people who sign up to be cryogenically preserved usually hope to be resuscitated rather than cloned.

Journal reference: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.0806166105.